With the perspective of detection and study of mechanisms by which environmental agents induce developmental disorders, an in vitro culture system of embryos during organogenesis phase of development in the rat (pregnancy day 11) paralleling in vivo development has been established. In this culture system, the embryos grow 3-4 times their original size in 24 hours as assessed grossly, and by DNA and protein content. The major organs, brain, neural tube, sensory organs, circulatory system, gut, liver and musculature in the cultured embryos differentiate comparable to that in vivo.